In small and medium size CFB boilers, typically having a capacity of about 300 MWe or less, there are usually from one to four particle separators, which are all arranged on one sidewall of the boiler. In large size CFB boilers, having a capacity of more than about 300 MWe, there are typically multiple particle separators arranged on each of the two opposite long sidewalls of the boiler. When all the particle separators are connected on the same sidewall of the furnace, or there is only one particle separator, it is known to arrange the back pass on the same side of the furnace as the separators, whereby the arrangement is known as an in-line construction. Alternatively, the back pass and the one or more particle separators arranged on one side of the furnace can be positioned on opposite sides of the furnace, whereby the construction is known as an over-the-top-construction, because the flue gas ducts, connecting the gas outlets of the particle separators to the back pass, conduct cleaned flue gas over the top of the furnace.
Large size CFB boilers, having multiple particle separators on each of the two opposite long sidewalls of the boiler, usually have a furnace with a rectangular cross section, in which the width of the long sidewalls is clearly larger than the width of the short sidewalls. Such large CFB boilers have, according to the prior art, a back pass arranged adjacent to a short sidewall of the furnace. The gas outlet tubes of the particle separators arranged on the same sidewall, the number of which being typically at least three, are connected to a common flue gas duct, which conducts the clean flue gases to the back pass. Because there are particle separators on both long sidewalls of the furnace, the flue gas duct system naturally comprises two flue gas ducts. Such flue gas ducts are then arranged parallel to the long dimension of the horizontal cross section of the furnace, either above the separators, or on top of the furnace. An example of a CFB boiler with flue gas ducts above the separators is described in the article “Milestones for CFB and OTU Technology—The 46 MWe Lagisza Design Supercritical Boiler Project Update”, presented at a CoalGen Conference in Milwaukee, Wis., in August, 2007.
The flue gas ducts of large CFB boilers of the type described above are fairly long, for example, more than thirty meters in the largest CFB boilers of today. Therefore, the flue gas ducts have to be well supported, in order to obtain sufficient stability and durability of the construction. According to an advantageous arrangement, disclosed in U.S. Pat. No. 7,244,400, the flue gas ducts are formed above the furnace, as extensions of the furnace walls. This arrangement provides a rigid and durable construction, which, to some extent, minimizes the problems related to the conventional construction of long flue gas ducts.
Each of the two flue gas ducts of a conventional large circulating fluidized bed boiler collects flue gas from, for example, three or four separators. Thus, the gas flow becomes, especially at the final sections of a flue gas duct, very high, and potentially eroding, unless the cross section of the flue gas duct increases towards the end. Such gradually widening flue gas ducts are, however, complicated constructions. Another possibility is that the long flue gas ducts have a constant cross-sectional area that is wide enough to maintain a sufficiently low flow velocity even at the end. Such construction increases the weight of the flue gas ducts and may cause problems due to the non-constant velocity of the flue gas flow.
The article “Recent Alstom Power Large CFB and Scale up aspects including steps to Supercritical,” presented at the 47th International Energy Agency Workshop on Large Scale CFB, Zlotnicki, Poland, on Oct. 13, 2003, shows a large CFB boiler having three particle separators on each of the long sidewalls, in which the outlet ducts of the particle separators on each side are connected together by a collecting channel and further to the back pass by a common flue gas duct, which flue gas ducts are connected to the centers of the collecting channels. This arrangement provides a complicated construction, which is, for example, difficult to support.